Re: Capacitive Ballasting

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "davep by way of Terry Fritz <twftesla-at-qwest-dot-net>" <davep-at-quik-dot-com>

	Malcolm speaks great wisdom.

	Such caps are designed to be fired 'occasionally, for
	24 to 36 shots, then discarded.  (OK: the things DO
	get recycled....)   Electrolytic caps will, as
	Malcolm notes, overheat if used continuously with major
	AC power/current.

	I HAVE SEEN THIS HAPPEN.

		Large, Dangerous, Bang.

	best
	dwp

Tesla list wrote:
> 
> Original poster: "Malcolm Watts by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>
> 
> Hi Alfred,
> 
> On 24 Feb 2002, at 22:26, Tesla list wrote:
> 
> > Original poster: "Alfred Erpel by way of Terry Fritz
> <twftesla-at-qwest-dot-net>" <alfred-at-erpel-dot-com>
> >
> >
> > Howdy all,
> >
> >  I was wondering about the merits of capacitive ballasting a transformer
> > with electrolytic photo capacitors since you can get them for free from
> > disposable cameras and they have high enough capacitance in a small enough
> > footprint to be practical.
> >
> > ASCII art warning -- make sure you have on a fixed width font. I use
Courier
> > New.
> >
> >
> >
> >                    electrolytic
> >          diodes     capacitors
> >    -------->|--------|+ -|-------------o  o-----
> >   |     |                   |          o||o
> >   |     |                   |          o||o
> >   |      --|<--------|+ -|--           o||o
> >   |                                    o||o
> > |A C|                                  o||o Xformer
> >   |                                    o||o
> >   |      -->|--------|+ -|--           o||o
> >   |     |                   |          o||o
> >   |     |                   |          o||o
> >    --------|<--------|+ -|-------------o  o------
> >                    electrolytic
> >          diodes     capacitors
> >
> >
> > Would the above circuit have any problems? Is this a stress for a good
> > electrolytic?
> > X(c) = 1 / (2*pi*f*capacitance)
> > At .000160 F and f=60 Hz, a typical photo electrolytic yields a
reactance of
> > 16.578 ohms. Putting 4 in series would yield 66.315 ohms which would limit
> > 115 volts to 1.734 amps. I know one thing to worry about would be that you
> > aren't around resonance with the transformer's primary. How would each leg
> > of the circuit "see" the capacitance? For instance, if the circuit above
> > just had four 160 uF capacitors (one for each diode), for calculating
> > reactance would you use 80 uF in the reactance equation or 320 uF? Would it
> > even work?
> 
> An interesting question. Personally I wouldn't use electrolytics in
> AC operation. But before commenting further, there is a serious
> problem with your circuit. You will end up with DC charged capacitors
> and no circuit current once they've charged. The correct way to do
> this is:
>         +            +
> ---------||--------||-----------
>     |         |         |
>     |         |         |
>     ----|<----+---->|----
 
>     The diodes are to prevent serious reverse polarity charging once
> the caps are emptied.
 
>     I have a reservation about using photoflash caps based on their
> physical size. I don't think they are good for high RMS currents. One-
> off high peak currents, yes. I imagine they would be the standard in
> filter caps if they were up to it as everyone would like to shrink
> high value, high voltage caps to zero size.

> Regards,
> Malcolm